Expression of theAgrobacterium tumefaciens virulence (vir) gene is known to be dependent on host plant phenolic compounds. TheA. tumefaciens strain A348 (pSM358) harbouring avirE::lacZ fusion plasmid was used to detect the ability of 13 synthetic acetosyringone, acetovanillone, syringaldehyde and syringic acid β-glycosides to induce virulence. The activity of the reporter β-galactosidase was detected by spectrofluorimetry using 4-methylumbelliferyl β-galactopyranoside as substrate. Acetosyringonyl β-l-fucopyranoside was the most active monoglycoside tested; even at high concentration this compound was devoid of toxic effects. However, monoglycosides were less activevir inducers than free acetosyringone. In contrast, the β-maltoside of syringaldehyde showed higher activity than the free phenol at high concentration. The activity of such glycosylated inducers may be related to specific sugar receptors on the bacterial cell surface.